This invention relates to polishing pads used for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectric/metal composites and integrated circuits. It particularly relates to the surface texture of such pads.
Polishing generally consists of the controlled wear of an initially rough surface to produce a smooth specular finished surface. This is commonly accomplished by rubbing a pad against the surface of the article to be polished (the workpiece) in a repetitive, regular motion while a solution containing a suspension of fine particles (the slurry) is present at the interface between the polishing pad and the workpiece. Commonly employed pads are made from felted or woven natural fibers such as wool, urethane-impregnated felted polyester or various types of filled polyurethane plastic.
The polishing rate for such a system is determined by the pressures and velocities employed as well as the concentration of fine particles in contact with the workpiece at any given time and the chemical reactivity of the slurry. To increase polishing rates, patterns of flow channels are commonly cut into the surface of polishing pads to improve slurry flow across the workpiece surface. Additionally, the reduction in the contact surface area effected by such patterning provides higher contact pressures during polishing, further enhancing the polishing rate. Typical examples of textured pads are grooved, embossed and perforated pads sold commercially by Rodel, Inc. of Newark, Del. under the trade names Suba and Politex. A typical grooved or embossed pattern is a 0.100 inch square grid of 0.008 to 0.014 inch depth recesses.
The texture described in the related art is generally of a fixed large dimension. Texture spacings or depths are of a dimension clearly visible to the unaided eye, i.e. they may be termed macrotexture. In most related art, macrotexture consists of a regular geometrical array of grooves or spaces to create simple polygonal, spiral, lined, cross-hatched or circular areas of raised relief. A typical example of this is U.S. Pat. No. 2,701,192 which discloses the use of concentric, radial and cross-hatched grooves of regular spacing to improve slurry uniformity. A more recent patent, U.S. Pat. No. 5,232,875, shows a regular array of perforations through the pad which enable slurry to flow up through the pad to the interface between the workpiece and pad. U.S. Pat. No. 5,177,908 shows patterns of grooves or perforations in the pad surface which vary in size or density from the center to the circumference of the polishing pad for the purpose of providing a constant, or nearly constant, surface contact rate to a workpiece.
Generally macrotexture is applied prior to the use of a pad, however, U.S. Pat. No. 5,081,051 describes a process for continuously forming a plurality of circumferential macrogrooves during the polishing process. As stated in the specification (col 3, lines 63-64) the pad employed is specifically one which itself is "capable of absorbing particulate matter such as silica or other abrasive materials", i.e., the pad possesses a preexisting porosity or surface texture.
The only related art which teaches the use of grooves and patterns of different sizes simultaneously U.S. Pat. No. 5,216,843 which discloses a method for continuously producing small scale grooves on the surface of a pad during the polishing process. As stated in the specification of this patent (col 4, lines 23-25) the pad employed is specifically one which is "capable of transporting abrasive particulate matter such as silica particles", i.e., a second type of microtexture is added to the already existing porosity or surface texture of the pad. These microgrooves are cut across a raised region between larger preformed grooves (macrogrooves) exclusively to facilitate slurry transport. Typical macrogrooves shown are a plurality of circumferential concentric grooves approximately 0.3 mm deep and 0.3 mm wide cut into the surface of a circular polishing pad. During pad rotation a conditioner arm having a diamond tip is swept across the pad surface in an oscillating radial fashion during polishing to produce a series of shallow radial microgrooves across the pad surface. These microgrooves, approximately 0.04 mm wide by 0.04 mm deep, facilitate slurry transport in the region between the macrogrooves.
While U.S. Pat. No. 5,216,843 recognized both macro- and microtexture as being contributors to slurry transport, no teaching was made as to any interrelation of the dimensions or concentration of each. Thus, while a range of macrogroove densities of between 2 and 32 macrogrooves per inch was specified, no range of microgroove densities is given. Moreover, the inventors specifically mentioned that the presence of the macrogrooves is optional and that radial microgrooves by themselves are sufficient for slurry transport. In addition, the inventors specifically taught that the process is restricted to those pads which are capable of permitting transport of slurry particles on the pad surface. Such pads, typified by the preferred embodiment, an IC60 pad manufactured by Rodel, Inc. of Newark, Del., possess a well defined surface texture capable of transporting slurry, and the pads are capable of considerable polishing activity by themselves when neither macrogrooves or microgrooves are present. Indeed, as an example, IC60 pads are widely employed in the glass polishing industry in such an unmodified state with good effect.
All prior art polishing pads known to the inventors are composite or multiphase materials which possess an intrinsic microtexture as a result of their method of manufacture. The surface microtexture is derived from bulk non-uniformities which are deliberately introduced during manufacture of the pad. When cross-sectioned, abraded, or otherwise exposed, said bulk texture becomes a surface microtexture. This microtexture, which is present prior to use, permits the absorption and transport of slurry particles, and gives rise to polishing activity without further addition of micro- or macrotexture to the pad. Examples of the various classes of prior art polishing pads are as follows:
1. Urethane impregnated polyester felts (examples of which are described in U.S. Pat. No. 4,927,432) possess a microtexture derived from the ends of projecting fibers within the bulk composite, together with associated voids. PA1 2. Microporous urethane pads of the type sold as Politex by Rodel, Inc. of Newark, Del. have a surface texture derived from the ends of columnar void structures within the bulk of a urethane film which is grown on a urethane felt base. PA1 3. Filled and/or blown composite urethanes such as IC-series, MH-series and LP-series polishing pads manufactured by Rodel, Inc. of Newark, Del. have a surface structure made up of semicircular depressions derived from the cross-section of exposed hollow spherical elements or incorporated gas bubbles. PA1 4. Abrasive-filled polymeric pads such as those of U.S. Pat. No. 5,209,760 possess a characteristic surface texture consisting of projections and recesses where filler grains are present or absent.
In contrast, solid homogenous sheets of polymers such as polyurethane, polycarbonate, nylon, or polyester have been demonstrated to have no polishing activity, and are, in consequence, not employed as polishing pads.
As a consequence of the need for a composite structure, the process for manufacturing prior art polishing pads is quite complicated relative to the manufacture of solid homogenous plastics of equivalent dimensions and thicknesses. In addition, there is considerable variability in the structure of prior art polishing pads as a consequence of their manufacture. Thus, for example, variability in the density of the felt for pads of class (1) above, or variations in filler density for pads of class (3) above will cause a corresponding variation in surface texture and, therefore, in polishing performance. This variability is well known to those skilled in the art and is one of the biggest deficiencies of prior art polishing pads.
Moreover, all prior art polishing pads known to the inventors possess significant polishing activity without additional macrotexture or microtexture being present, i.e. both are added as a refinement or improvement to performance, and are not required for polishing activity.
Accordingly, it would be highly desirable to provide a surface texture which did not rely in any way on preexisting inhomogeneity in the bulk material. This would allow employment of previously unusable but highly desirable materials as polishing pads, with corresponding improvements in polishing activity, performance stability, performance variability, and cost.